Transducers are widely used in measurement systems. The term transducer refers either to a sensor or an actuator. “Sensor” normally refers to a device that detects a change in a physical property due to a physical stimulus and turns that stimulus into a signal, which can be measured or recorded. “Actuator” refers to a device that accepts an input stimulus and converts it into a different kind of output stimulus. Then, the term “transducer” normally refers to a device that transfers energy of one kind to another in the same or a different form and includes either a “sensor” or an “actuator”. Thus the sensor is considered to be the sensing element itself and the transducer is considered to be the sensing element plus any associated circuitry for transmitting the measured physical change to a remote location. Transducers can be used to test the state of a mechanical or physical system or some portion of that system.
Through the use of transducers the properties of a system can be measured by observing the change in the properties of another system. For example, the absorption of ultraviolet light in some chemical compounds can be measured by directing a specific incident spectrum of light onto a compound and measuring the intensity of reflected light as a function of wavelength. The sensor provides the measurement and the transducer converts it to an electrical signal that is representative of the measurement. From the signal a computer analyzes which wavelengths are absorbed and which are reflected while also taking into account any other factors.
There are a number of types of transducers. Generally transducers are classified according to the physical effect that they use or according to the property that they measure. For instance, the effect that they use might be piezoelectric or photovoltaic effect and the property that they measure might be a measurement of length or temperature. Although transducers are often used in mechanical and physical systems, the amount of confidence in the result of a measurement taken through a sensor is greatly increased when all the factors influencing that measurement are monitored.
There is a class of transducers that are called self-generating transducers, which include thermocouples, piezoelectric, and photovoltaic transducers among others. These transducers do not require a power supply because they generated a signal internally. A thermocouple, for example, produces a change in voltage in response to a temperature difference and a piezoelectric material generates a charge that can be measured when it is stressed. There are a number of piezoelectric devices including accelerometers.
Piezoelectric devices generally are particular types of crystals that develop a charge when stressed in a specific direction. The charge that develops in a piezoelectric transducer is proportional to both the piezoelectric constants of the material and the applied stress on the piezoelectric device. This piezoelectric constant depends on the mode of operation employed including bend, shear and compression. Quartz crystals are used as piezoelectric devices in some systems, but manufactured ceramics, such as Lead-zirconate-titanate (PZT) ceramics, are also widely used in piezoelectric transducers because they have a higher piezoelectric constant and therefore a higher charge output. Many piezoelectric transducers have built-in charge amplifiers. This greatly reduces problems with interference and low signal levels.
Generally, piezoelectric accelerometers and other transducers that are being used as sensors are meant to operate for long periods of time with minimal maintenance. It is important that if there is a failure or degradation of the piezoelectric material or transducer that the operator know this as soon as possible. For instance, if there is a failure of the bond between the structure being monitored and the piezoelectric accelerometer, it would be important for an operator to know this as soon as possible since any data collected subsequent to the failure would be unreliable. If there is any internal mechanical failure of the piezoelectric accelerometer, the device is useless and needs to be replaced immediately.
In the past, failure or degradation and other related properties have been difficult to ascertain without specific testing equipment and without taking apart the machine on which the transducer was installed. With the advent of microcircuitry and embedded sensors, this is extremely expensive and difficult to accomplish without destroying the transducer and possibly other parts of the monitoring device as well as the body being monitored. Thus any sensor data must be accurate and reliable if it is to form the basis of the very important and costly condition-based maintenance decisions.
Accordingly, there is a need for improvement in the art to enable transducers to be tested in-situ and specifically, there is a need for an apparatus and method to allow in-situ testing of transducers such as piezoelectric transducers and accelerometers that convert mechanical strain to an electric signal. There is a particular need for a self-testing transducer circuit. It is to such improvements that the present invention is directed.